Deterioration reactions

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

The moisture content of a given component is in turn determined by the existing vapor pressure of water (or relative humidity) and is described by an isotherm for that component. Furthermore, it seems plausible to assume that the deterioration of the food is a function of the moisture content of the particular sensitive component (or components) that is involved in the deteriorative reaction. The over-all moisture content is thus of little significance. 

Some polymer deterioration reactions occur without loss in molecular weight. These include a wide variety of reactions where free radicals (most typical) or ions are formed and cross-linking or other nonchain session reaction occurs. Cross-linking discourages chain and segmental chain movement. At times this cross-link is desired such as in permanent press fabric and in elastomeric materials. Often the cross-links bring about an increased brittleness beyond that desired. 

Physical preservation methods for foods, such as sterilization and drying, are associated with the application of heat. In these cases, because of its high temperature coefficient, the Maillard reaction becomes the dominant deteriorative reaction (1 2, 3). 

From these experiments general conclusions can be drawn with respect to an improvement of the quality of dried products. However, these investigations are based on the measurement of browning being the last step of a multistage deteriorative reaction which normally is already accompanied by the formation of off-flavors. 

It becomes clear that analytical methods based on the evaluation of the end products of deteriorative reactions will not be satisfactory. Therefore in our own experiments amino acid analysis of Amadori compounds and gas chromatography of volatile Strecker aldehydes were applied to detect the onset of the Maillard reaction well before detrimental sensory changes occurred. 

Four major objectives in the chemical modification of food proteins are (a) the blocking of deteriorative reactions (b) improvement of physical properties (c) improvement of properties related to acceptability, such as flavor and color and (d) improvement of nutritional properties. Modifications also may aid the physical separation from crude animal, plant, or microbial material and may inactivate or remove relatively small amounts of undesirable substances. There is considerable interest among food technologists in chemical modification as a means of changing the physical characteristics of proteins to give, e.g., better solu-... 

Aims To prevent deteriorative reactions of lysyl residues. 

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

The deteriorations and the deteriorative reactions of proteins have been studied by scientists in many different fields for many centuries. In order to give proper tribute to the almost ancient importance of proteins, it would be necessary to summarize the history of agriculture, medicine, food processing, and much of industry. Scientists and technologists have long recognized both the adverse and beneficial facets of deteriorative changes in proteins. 

The isolation, preservation, and analysis of proteins were among the primary areas of protein chemistry until the early 20th century. In nearly every step of isolating proteins, workers encountered the problem of preventing deteriorative reactions and, as a consequence, began to study the deteriorative reactions themselves. Many of these earlier studies of deteriorative reactions have now been described >n quantitative chemical terms, but many still elude the efforts of current workers using modern techniques. 

The immensity of this subject at first made it seem that an overview could only be done one of two ways, 1) essentially a many-page outline of the deteriorations, or 2) a selection of two or three deteriorative reactions and their coverage in a comparative and illustrative way. There were suggestions from several sources that a more illustrative coverage could be based on the... 

Approximately 150 different amino acid residues have been reported in proteins (1 5). At least half of these could undergo chemical deteriorations under the conditions of stress usually encountered. Many of these deteriorative reactions involve hydrolytic scissions, not only of peptide bonds but of the many different nonprotein substances added covalently to proteins postribosomally. These susceptible side chain groups are indole, phenoxy, thioether, amino, imidazole, sulfhydryl, and derivatives of serine and threonine (such as 0-glycosyl or O-phosphoryl), the disulfides of cystine, and, of course, the amides (such as asparagine and glutamine). With strong acid or alkali, other residues, such as serine and threonine, also are less stable. 

When proteins are deliberately treated with chemicals in order to derivatize them, the reaction conditions may also cause chemical deteriorative side reactions. Some of the more common ones are listed in Table III. Inspection of Table III shows that many of these effects are those found in deteriorative reactions... 

As is the case with some chemical changes occurring in biological systems, such as the blood-clotting cascade system, deteriorative reactions considered to have a beneficial effect are found in foods. For example, the Maillard reaction (lJ7,lj3) is used to produce flavors and colors in such foods as beverages and baked goods. Heat treatment (involving denaturation) has been found to increase the nutritional value of raw soybean meal by... 

Photooxidations are not normally considered a protein deteriorative reaction because they usually go unseen or are found only when purposely contrived, such as in the chemical modification of proteins (Figure 17). However, the possibility of their occurrence in foods, particularly those containing added dyes, should not be overlooked. Several of the important amino acid side chains are readily modified, including the sulfhydryl, imidazole, phenoxy, indole, and th1o ether (Figure 18). More general and detailed coverage 1s provided in another article in this volume (34). 

Although reversibility of the deteriorative reactions of proteins is usually not a practical possibility, particularly with mixtures of proteins and proteins in complex biological... 

The detection and determination of deteriorated proteins is probably one of the most difficult problems in protein chemistry today. The main reason is that it is usually easier to determine native properties, particularly when those properties include a biochemical function, than it is to determine a deteriorated and inactive protein. In addition, there are almost always no standards to use as a measuring stick for the deteriorated protein. When the deteriorative reactions are extensive and more than a fraction (30-40%) of the protein undergoes deterioration, the deterioration can frequently be monitored by determining the amount of native protein remaining, providing, of course, that the deteriorated protein does not interfere with the determination of... 

In spite of these many limitations, there are a number of approaches that have been successfully used for certain deteriorative reactions (Table XIII). The most obvious approach is to determine the biochemical or biological activity of a protein when the protein has such a property. The difficulty in these determinations is that the deteriorated protein may have a biochemical activity that is changed or attenuated without loss of activity. In such a hypothetically modified protein, a biochemical determination might show that there is only 50% activity present. However, such a determination could not differentiate between complete inactivation of 50% of the molecules or a 50% reduction in activity of 100% of the molecules by an attenuation rather than an inactivation. A different test would be necessary in order to "count" the number of biochemically active molecules. For this purpose, the enzyme chemist sometimes uses an active site titrant, which is a type of an affinity reagent (16). 

Pharmaceutical effects. Most pharmaceutical agents involve drugs which act either at cell receptor sites or directly on enzymes. In both cases proteins are involved. With many drugs, there is a man-directed deteriorative reaction to inactivate a particular target (43). Usually, the greater the specificity of inactivation, the more specific the drug. 

In relation to food proteins, chemical modification has been studied for several purposes, i.e. to block reactive groups involved in deteriorative reactions to improve nutritional properties, to enhance digestibility to impart thermal stability to modify physicochemical properties to facilitate study of structure-function relationships and to facilitate separation, processing and refining of proteins (1,2,10,19,20,24). 

Ideally, aging processes would be followed in ambient conditions, but the deterioration reactions are sufficiently slow so that finding techniques sensitive enough to follow them is difficult. 

Preparative Methods see Trichloro(cyclopentadienyl)titanium. Handling, Storage, and Precautions best handled as stock solution either in Et20 (ca. 0.1 M) or toluene (ca. 1.5 M), which must be protected from moisture and UV light. If handled under an inert atmosphere (argon), such solutions can be stored in a refrigerator (8 °C) for several months (possibly much longer) without deterioration. Reactions should be carried out in dry equipment and with absolute solvents under Ar or N2. 

Commercial alumina packing materials for HPLC have lower column efficiency values and lower specific surface areas (typically 100-250 m g ) which leads to a lower sample capacity. Another disadvantage of alumina is that the highly active surface has a tendency to catalyse deterioration reactions, which is particularly important for molecules which undergo base-catalysed degradation, because of the basic nature of the alumina surface. 

Free radicals are generated at the wood surface during irradiation. The rate of free radical formation is enhanced when moisture content increases from 0 to 6.3%. Electron spin resonance and UV studies on the behavior of free radicals generated and their interactions with oxygen molecules to form hydroperoxides revealed that free radicals and singlet oxygen play important roles in discoloration and deterioration reactions of wood surfaces. 

Fig. 3.71 Generalized deterioration reaction rates in food systems as a function of water activity (room temperature). Adapted from Fennema [9] and Heiss and Eichner[10] ...

The glass transition processes in foods may result from a rapid removal of water from solids. Based on that, e.g., the Tg values of anhydrous polysaccharides are high, and the food materials may decompose at temperatures below Tg (Kokini et al 1994 Roos and Karel, 1991b). The glass temperature transition affects viscosity, stickiness, crispness, collapse, crystallization, and ice formation, and can strongly influence deteriorative reaction rates. This provides a new theoretical and experimental framework for the study of food systems to unify structural and functional aspects of foods, described in terms of water dynamics and glass dynamics. 

The Stokes-Einstein Dgff is proportional to temperature both directly and through its effect on rj. Yet on an absolute scale, temperature increases are relatively small for deteriorative reactions in foods. For example, when increasing temperature from 20°C (293 K) to 45°C (318 K), the theoretical diffusivity by Stokes-Einstein increases only 8%. Since deteriorative food reactions typically increase by 200 to 1000% per 10°C increase (Qio = 2 to 5), a simple local translational diffusion model would be insufficient. 

Calorimetric studies on embryos by DSC showed an endothermic event attributable to a glass transition between — 60 and — 65°C. Thus, at storage conditions (5°C), the systems were well above the glass-transition temperature, with most of their solutes in solution or in supercooled liquid state. At such a high proportion of water, metabolic activity and deteriorative reactions involved in the short life span of this seed are not precluded by mobility restrictions. The nonstored seeds presented the relative lowest values of freezable water and they were selected for the studies on cryoconservation. 

---